Why is it the killers fault for killing someone ? I assume that is rhetorical a statement. If it wasn't i suggest getting your head checked up on.
Under what circumstances other then war would justify prematurely taking a human life. Don't replay if someone abused your mum , sister , even then taking that persons life would not be justified by the fact that: everyone dies at one point. Which is what you stated in the OP. If you do in fact take a life before nature intended, then you should suffer the repercussions.
Also, when mourning over the dead, you do not only feel sorry for the victim but also the dead's family and close friends, whose lives have dramatically changed due to the loss of someone close to him.
If you feel that feeling sorry for someone who lost a big part of their life is annoying or unnecessary then you need get your head checked out again, for feeling sorrow and sadness for others are normal when associated with death.